In Africa, where the Nyaga Ratu River meets the sea, an island tribe known as the Aquatanians dives for pearls, fishes and worships Balu, a false god created by the power-hungry high priest Palanth. Balu appears to the Aquatanians as a large, mute figure who resides in a temple on top of a rocky island, which Palanth has declared off-limits to anyone but himself. In reality, the "god" is Varga, a greedy trader in a plaster costume, who covets the pearls that the tribe gives him as tribute. As Mara, a beautiful Aquatanian who has been chosen to be Balu's bride, is being escorted to her "groom" during her wedding ceremony, she sees through the deception and flees in horror. Mara, who loves the banished Tiko, dives into the bay and heads upstream, eventually coming to the jungle home of wild man Tarzan and his mate Jane. Seeing the mermaid-like Mara swimming near his river fishing spot, Tarzan jumps in and chases her through the water until she collapses from exhaustion. Tarzan takes Mara to Jane and Benji, a singing postal carrier, who nurse her until she is well enough to tell her story. Out of gratitude, Mara gives Jane and Tarzan a large black pearl, which Jane then decides to donate to the area's new reform-minded commissioner. While Benji takes the pearl to the commissioner in Nyaga, Palanth sends some men to search for Mara. Eventually, Mara is captured, but Tarzan is able to follow the boat carrying her back to Aquatania. After Mara is returned to the tribe, Tarzan sneaks into Balu's temple and spies Varga shedding his costume and leaving the island. Benji, meanwhile, has shown the inspector-general and the commissioner the pearl and guides the suspicious commissioner to Tarzan's home. There Jane, who has met up with the lovelorn Tiko, tells the commissioner Mara's story, and she, Tiko, Benji and the commissioner then leave for Aquatania. As they near the island, Palanth orders that they be brought to the temple and there begins to question them. To Palanth's surprise, Balu suddenly appears and silently commands that Mara be freed and Tiko, forgiven. As Balu orders that the "intruders" be allowed to leave unharmed, Jane realizes that Tarzan is inside the costume. Unable to expose Tarzan, Palanth is forced to release the group, which then prepares to celebrate Mara and Tiko's wedding. In Nyaga, meanwhile, Varga is questioned by the inspector-general and hurries back to Aquatania. During Mara and Tiko's pre-nuptial celebration, Varga suddenly appears as Balu and, through Palanth, orders that the intruders be killed. Tarzan, however, is able to fend off Varga's thugs underwater and, after fighting an octopus, swims back to the temple and exposes Balu as a fake. The angry Aquatanians then dispose of Varga and Palanth and resume their happy celebration.

The opening credits for this film read: "RKO Radio Pictures, Inc. presents Edgar Rice Burroughs' Tarzan and the Mermaids ." The onscreen crew credits are divided into two sections, the first listing the American crew, the second, labeled "Associate Production Personnel," listing the Mexican. According to HR , exterior scenes for the picture were shot in Acapulco and the Mexico City area. A pre-production news item announced that additional shooting was to be done in Florida, but it is not known if filming ever took place there. Many reviewers commented on the fact that, although the story is set in Africa, the locations and actors are clearly Latin American. Although onscreen credits say "and introducing Linda Christian," the actress had appeared in three previous films. According to a 31 Jul 1947 HR news item, Angel García, Johnny Weissmuller's double, was killed while performing a high dive in the film. HR also noted that, to cover his sunburned nose, Weissmuller wore screen makeup in this film for the first time in his acting career.
With a budget in excess of a million dollars, Tarzan and the Mermaids was the most expensive Tarzan movie produced to date, according to HR . John Laurenz, who plays "Benji" in the picture, composed his own calypso songs for the production. Although a HR production news item announced two of the song titles as "I'm Taking a Letter to My Friend Tarzan" and "Farewell, Fair Mermaid," it is not known if either of those songs was used in the final film. Weissmuller, who was forty-four-years-old in 1948, made his last appearance as "Tarzan" in the picture. In 1949, Lex Barker took over the role in RKO's Tarzan's Magic Fountain (see below). Modern sources add Lilia Prado, Silvia Derbez, Magda Guzmán, Ana Luisa Peluffo and Salvador Godínez to the cast. For more information about the "Tarzan" series, consult the Series Index and see the above entry for Tarzan Triumphs and the entry for Tarzan, the Ape Man in AFI Catalog of Feature Films, 1931-40 .

The American Film Institute is grateful to Sir Paul Getty KBE and the Sir Paul Getty KBE Estate for their dedication to the art of the moving image and their support for the AFI Catalog of Feature Films and without whose support AFI would not have been able to achieve this historical landmark in this epic scholarly endeavor.